Increasingly in many industries, parts formerly made of metal or wood are molded from plastics. This trend is especially strong in the automobile industry where due to the increasing emphasis on the reduction of body weight to improve operating efficiency and reduce cost, an increasing variety of components or parts of the automobile are being molded from plastics. Among the parts made in this way are those suited for the interior trim such as door and window trim elements, door panels, dashboard panels, steering columns and transmission housings, and armrests, for example, and quite recently this trend is even being extended to exterior trim components.
In addition to the automotive industry, cabinets and housings for televisions, various items of electronic equipment, and industrial equipment are produced in this manner.
In many if not most instances, a preference exists for a matt or flat finish on the grain-finished parts or components in question for various reasons, including a reduction in light reflection which can be objectionable in certain locations and better durability since a glossy finish is more susceptible to damage by scratching or abrasion which may be hardly noticeable on a roughened or matt-finish surface. Generally, these parts and components are molded by injection molding which can now be satisfactorily carried out even for relatively large objects, and in order to impart to the molded product the desired finish, the interior surface of the mold is first grained or pebbled to create a pattern generally of peaks and valleys and the grained surface is then treated to roughen the surfaces of the individual peaks and valleys and create the desired degree of mattness or flatness. Such treatment can be done in several known different ways, e.g., by sand-blasting with the size of the grit being selected as needed or by reducing the severity of an excessive initial degree of roughness to a desired extent by a kind of "bead peening" with glass beads of the required size.
In the injection molding of such matt-finished parts and components, however, it becomes virtually impossible to achieve consistently a matt-finished product free of visible defects, particularly where the part being molded is large, such as door panels, door post covers, dashboard panels and the like. Such defects can take a variety of different forms such as so-called "water marks", formed by water droplets condensed in the mold during cooling after the previous injection, "flow lines" caused by temporary discontinuities in the plastic flow through the mold, glossy spots, lines or off gloss areas, etc., all of which are manifested as a visible blemish in the otherwise generally uniform matt finish over the exterior surface of these grained parts. In addition, the mold surface may not be uniform around so-called "knock-out pins" or defined openings in the mold, while the mold surface itself with repeated use undergoes wear at particular points, creating surface irregularities. Such surface blemishes are, of course, objectionable under the standards of the manufacturer of the final product using such parts, giving the impression to the customer of poor quality and shoddy workmanship, and the industry has been faced with the alternatives of either discarding these defective parts or else applying a surface coating over the entirety of the exposed surface of the defective parts so as to obliterate or conceal the defect. The practice in the latter case has been to provide a full-scale spray paint booth wherein the entire surface of the defective part is sprayed, often after application of a prime coating, with a coating pigmented to a color corresponding to that of the "color-coded" part in question. Each manufacturer usually has a set of standard colors or "color codes" which can be matched by the bulk supplier of the pigmented plastic to be molded or by other suppliers, and the parts are molded to match these colors, being referred to in the art as "color coded". Plastics, such as the polyolefins, e.g., polyethylene and polypropylene, are notoriously difficult to coat due to their recognized resistance to virtually all common solvents except under extreme conditions, and consequently, the application of a prime coat has been virtually essential to insure proper adhesion of the ultimate top coating, the latter being formulated to provide a flat or matt effect following the well developed principles of paint formulation. On the other hand, ABS for example by nature finishes too "glossy" to meet the current gloss standards of the automotive industry.
The provision of paint booths with their accessory equipment involves a substantial capital investment and a variety of top coating formulations must be kept in stock to allow for matching with the variously color-coded parts or components. The treatment of the blemished and unacceptable parts and components in this fashion thus adds materially to the ultimate cost of their production, and it would be desirable if there were available a less complicated and expensive technique for restoring surface blemished matt-finished grained molded color-coded plastic parts to an acceptable appearance.